This disclosure relates to the field of well logging instruments. More specifically, the disclosure relates to well logging instruments having sensors disposed in a pad or arm extending laterally from the instrument housing to make contact with the wall of a wellbore or the interior of a pipe, casing or tubing.
Oil and gas exploration and production uses certain types of instruments or tools lowered into wells drilled through subsurface formations. Such instruments may be lowered into and withdrawn from a well through a conduit such as drill pipe, tubing and casing.
Among these tools, certain types of such well logging tools require having sensors applied against the borehole wall (or the inner wall of, e.g., a casing) to obtain good quality measurements. Such sensors may be referred to as “wall contact” sensors.
Wall contact sensors, for example and without limitation, micro-resistivity, dielectric, ultrasonic, wheel and nuclear sensor are often fragile and it is important to protect the sensors during conveyance through a wellbore, in particular one of the above mentioned types of pipe, to avoid their destruction or their unnecessary wear.
Wall contact sensors are typically spring loaded and have an active (power operated) retraction system. The retraction system may comprise hydraulic pumps and motors, or electric motors, making them complex assemblies.